Glass-silicone resin-stretched acrylic resin laminates

ABSTRACT

Transparent laminates comprising at least one external glass sheet and at least one stretched acrylic plastic sheet which are bonded together by a nonrigid, deformable silicone resin interlayer to prevent the difference in the coefficient of thermal expansion between glass and stretched acrylic plastic from causing undesirable structural properties.

United States Patent Inventors Nelson E. Burrin Fullerton; Ralph C.Shelton, Buena Park, both of Calif. App]. No. 650,375 Filed June 30,1967 Patented Nov. 2, 1971 Assignee Swedlow, Inc.

Garden Grove, Calif.

Continuation-impart of application Ser. No. 437,916, Mar. 8, 1965, nowabandoned.

GLASS-SILICONE RESIN-STRETCHED ACRYLIC RESIN LAMINATES -5 Claims, 2Drawing Figs.

US. Cl 161/193, 156/99,161/165,161/208 Int. Cl B32b 17/06 50 Field ofSearch 161/165, 193,208; 156/106, 99

[56] References Cited UNITED STATES PATENTS 3,160,551 12/1964 Buetow eta1. 161/208 3,312,587 4/1967 Wilson 161/193 3,261,739 7/1966 Porter161/193 3,310,458 3/1963 Mattimoe et a1 161/193 3,388,035 6/1968Mattimoe et a1 161/193 Primary Examiner-William J. Van BalenAttorney-Lyon and Lyon ABSTRACT: Transparent laminates comprising atleast one external glass sheet and at least one stretched acrylicplastic sheet which are bonded together by a nonrigid, deformablesilicone resin interlayer to prevent the difference in the coefficientof thermal expansion between glass and stretched acrylic plastic fromcausing undesirable structural properties.

GLASS-SILICONE RESIN-STRETCIIED ACRYLIC RESIN LAMINATES. Thisapplication is a continuation-in-part of Ser. No. 437,916, filed Mar. 8,1965 now abandoned, the disclosure of which is expressly incorporatedherein by reference.

BACKGROUND OF INVENTION It has long been recognized that it would bedesirable to construct transparent laminates from glass and stretchedacrylic plastic sheets such that an external glass sheet could functionto provide maximum abrasion resistance and the plastic sheet couldfunction as the load-bearing member. Stretched acrylic polymers arepreferred for use in such laminates and have been consideredparticularly desirable by aircraft manufacturers for use as windows andthe like.

However, the temperature conditions to which such laminates aresubjected in use, particularly when used in aircraft, cover a wide rangeof temperatures, e.g., 65 F. to +200 F. and higher. Thus differences incoefficient of thermal expansion between the sheets which comprise thelaminate becomes highly important since a large difference will resultin failure of the laminate unless means for preventing such failure arepresent. Because of the significant difference between the coefficientof thermal expansion of glass and that of plastic materials generallyused to prepare transparent laminates, prior art workers have beenrestricted to the use of glass-to-glass structures or plastic-to-plasticstructures such as those disclosed in U.S. Pat. No. 3,135,645. For thisreason, it has not previously been possible to produce a laminatestructure having one surface which has the high abrasion resistance ofglass in combination with another surface having the.

strength characteristics of a rigid plastic sheet.

The present invention makes it possible to form laminates from sheetshaving substantially dissimilar coefficients of thermal expansion whichare capable of withstanding the shear stresses caused by thisdissimilarity when such laminates are exposed to different temperatureswithout fracture or delamination. More specifically, this inventionmakes possible the combination of glass surfaces which provide anexcellent base for electrically conductive coatings which may be usedfor deicing or defogging with stretched acrylic plastic sheets such asoriented acrylics which, in addition to strength properties, provideexcellent optical quality and permit substantial weight decreaseswithout sacrifice in strength into an article which possesses greatstructural integrity when subjected to substantial temperature changes.

SUMMARY OF THE INVENTION Broadly, the laminates of the present inventioncomprise substantially rigid skins of glass and stretched acrylicadhered together in spaced-apart relation by means of at least one layerof tough, substantially nonrigid deformable silicone resins. Preferably,these laminates are prepared by casting a layer of tough, substantiallynonrigid, deformable silicone resin between the above-mentionedspaced-apart, tough, hard, substantially rigid skins.

It is a primary object of the present invention to provide laminatesmade from glass and stretched acrylic sheets having dissimilarcoefficients of thermal expansion which laminates are capable ofwithstanding widely varying temperature conditions without failure.

It is another object of the present invention to provide laminatescomprising a silicone interlayer which does not become rigid atextremely low temperatures.

It is another object of the present invention to provide laminatescomprising glass and stretched acrylic having highly dissimilarcoefficients of thermal expansion in combination with a silicone resininterlayer.

Other objects and advantages of the present invention, it is believed,will become apparent from the following detailed description of specificembodiments thereof.

DESCRIPTION OF PREFERRED EMBODIMENTS In the drawings:

FIG. 1 is a perspective cross-sectional view of a laminate of thepresent invention wherein one outer skin comprises glass and the otherouter skin comprises a stretched acrylic plastic.

FIG. 2 is a perspective cross-sectional view of a laminate of anotherembodiment of the present invention wherein both outer skins compriseglass and are combined with an inner skin comprising a stretched acrylicplastic.

In FIG. 1, outer skins 11 and 12 are illustrated as being bondedtogether by an intermediate layer 13. Skin 11 is glass and skin I2 is arigid stretched acrylic plastic as contrasted with intermediate layer 13which is a flexible silicone resin.

In FIG. 2, outer skins 14 and 15 are bonded to an internal structuralmember 16 by interlayers l7 and 18. In this embodiment, outer skins l4and 15 comprise glass while internal structural member 16 comprises a:rigid stretched acrylic material. Interlayers l7 and 18 comprise aflexible silicone resin.

The stretched acrylic plastic material which is used as a skin 12 or aninternal structural member 16 is a tough, hard, substantially rigidcomposition which is capable of being machined or molded in order toprovide a desired shape to the laminate. Furthermore, the stretchedacrylic is uniquely adapted to forming a tenacious bond to the flexiblesilicone resin which will be cast between at least two skins to form alaminate.

The stretched acrylic plastic materials may be any of the known rigid,resinous compositions familiar to those skilled in the art of aircraftglazing, such as stretched polymethylmethacrylate.

The material which comprises interlayers l3, l7 and I8 is a flexiblesilicone resin. Such resins may generally be cured at room temperatures.The silicone resins contemplated for use in the present invention mustbe capable of curing to a tough, nonrigid, deformable, resinous layerand must readily adhere to the glass and stretched acrylic. Thesesilicone resins may require the addition of catalysts for curing or maybe cured upon exposure to the atmosphere.

In general, the silicone resins preferred for use in the presentinvention are room temperature curing silicone resins which are pourableat room temperature, which are transparent and which cure in the absenceof the atmosphere upon the addition of catalyst and are based on one oftwo types of chemistry, but have certain features in common. Thesematerials generally are free of filler and. generally consistsubstantially of organosilicon materials with only a minor amount ofcatalyst. One type of such silicone resins uses as a cure mechanism theaddition of an Sil-I linkage of one organopolysiloxane molecule acrossthe double bonds of olefinically unsaturated radicals attached toanother organopolysiloxane molecule. One illustration of this type ofreaction is found in example 26 of U.S. Pat. No. 2,823,2l8, which patentis incorporated by reference herein, wherein the SiH groups of amethyl-hydrogen siloxane homopolymer are added across the vinyl groupsof a dimethylsiloxane-methylvinylsiloxane copolymer. In this patent. anisopropanol solution of chloroplatinic acid is used as a catalyst forthe reaction. Elemental platinum is disclosed as a suitable catalyst forsimilar reactions in U.S. Pat. No. 2,970,150, which patent isincorporated by reference herein. In a second type of curing system, thecondensation of silicon-bonded hydroxyl groups is relied upon to effectconversion of the liquid organopolysiloxane to the solid form. Ingeneral, this condensation is catalyzed by either acidic or basiccatalysts depending upon the particular system employed. Such a curingsystem is disclosed in U.S. Pat. No. 2,843,555, which is incorporated byreference herein, wherein the curing agents are metallic salts oforganic carboxylic acids. Another illustration of the same type ofcuring system is disclosed in French Pat. No. 1,320,767, which isincorporated by reference herein, and which describes the use of variousmetal salts as well as basic materials such as quaternary ammonium andquaternary phosphonium compounds, as well as catalysts systems whichcomprise both an amine, such as dimethylsoya amine and an epoxy compoundsuch as phenyl glycidyl ether.

All of the room temperature curing silicone systems contemplated for usein the present invention are characterized by viscosities in the rangeof several thousand centipoise where the material is readily pourable.These compositions cure to the solid, tough, nonrigid, deformable,elastic, state at room temperature or moderately above, subsequent tothe addition of catalysts. Other patents which disclose various types ofroom temperature curing silicone resins include U.S. Pat. Nos.2,894,930, 2,915,497 and 3,020,260, as well as Belgian Pat. No. 614,771,all of which are incorporated by reference herein. Commerciallyavailable silicone resins of the type suitable for use in the presentinvention are RTV-602, RTV-603 and RTV-6l marketed by General ElectricCompany and Sylgard 182 and 184 marketed by Dow-Corning Corporation.

Another resin for use in this invention is a polymer system in the classof Sill-olefin curing silicones. The basic feature of this system is theuse of two different silicone polymers in which the average molecule ofeach polymer contains either a silicon-bonded hydrogen or asilicon-bonded olefinically unsaturated radical. In the mixture ofmaterials one polymer has a silicon-bonded olefinically unsaturatedgroup and in one polymer the average molecule contains silicon-bondedhydrogen atoms. Generally, the number of silicon-bonded olefinicallyunsaturated groups is about equal to the number of silicon-bondedhydrogen atoms. In this case, the polymers involved aremethylvinylpolysiloxanes and methyl hydrogen polysiloxanes and thecatalyst is a compound of platinum.

The laminates of the present invention will include at least threecomponents including at least two outer skins, one glass and onestretched acrylic, and at least one intermediate silicone resin layer.In addition, a multiplicity of plies may be used to provide desiredproperties. The exterior skins may be of any desired thickness and maybe as thin as one sixty-fourth inch or as thick as one-half inch ormore. As shown in FIGS. 1 and 2, the stretched acrylic skin or internalstructural member will generally be somewhat thicker than the glassskin, but certain uses may make different relationships desirable. Thethickness of the silicone resin intermediate layer is of substantialimportance and this layer must be thicker than a mere adhesive. Forexample, the intermediate silicone layer should be at least onesixty-fourth inch thick in order to withstand stresses created bychanges in the temperature to which the laminate is exposed and may beas thick as three-fourths of an inch. Preferably, the silicone resininterlayer will be from onesixteenth inch to one-fourth inch thick.

The laminates of this invention may be prepared by fabricating the skinsin any known manner. For example, skins may be shaped from previouslyprepared materials such as various resins and glass by heating them andshaping them to a form. Such skins may also be prepared by molding in acustomary mold at customary molding temperatures. In preparing thelaminates, the prepared skins are held in spaced-apart relationship bymeans of various spacers or gaskets which maintain a cavity between theskins which is substantially equivalent in depth to the desiredthickness of the silicone resin interlayer. The spacers should be inertto the silicone resin interlayer material and may be made from materialssuch as plastic coated metal, polytetrafluoroethylene, etc. Thereafter,the silicone resin may be poured or forced into the cavity between theskins after which the laminate assembly is sealed and cured.

The conditions of curing, e.g., time, temperature and pressure are knownin the art and will depend on the silicone resins being cured.Generally, the temperature will be from room temperature to moderatelyabove, the time from a few minutes to several hours and the pressurewill range from atmospheric to 25 p.s.i. or more.

The present invention is further illustrated by the following examples:

EXAMPLE 1 A 12 inch X 12 inch flat panel laminate was made in thefollowing manner to produce a composite structure comprising a 0.250inch tempered soda lime glass sheet of the type normally used asstructural elements in laminated glass windshield application foraircraft transparencies, a 0.125 inch Sylgard 182 silicone resin (atwo-component pourable polymer) interlayer, and a 0.450 inch sheet ofstretched acrylic resin (Swedlow 350). The glass and acrylic sheets werecleaned. The glass sheet was mounted to aflat vacuum tooling plate ofapproximately the same size as the stretched acrylic sheet. The acrylicsheet was backed with tempered soda lime glass sheet to support theacrylic sheet during the subsequent casting and curing operation and toprevent contamination or deformation of the outer optical surface of theacrylic sheet. An internal tubing, elastomeric gasket and solid shimwere used about the periphery of the glass and acrylic sheets toconstruct a casting volume comprising about 0.125 inch thickness and theentire assembly was held together by means of C- clamps. The assemblywas placed in a vertical position with an unsealed area at the top.Thereafter, the interlayer casting resin was prepared by mixing thesilicone resin with l l phr. of Sylgard 182 curing agent. Entrapped airwas removed from the mixture by applying 25 inches of vacuum andmaintaining the vacuum until all bubbles collapsed. This air-freemixture was poured into the panel assembly and was allowed to standvertically until all entrapped air had migrated to the surface andescaped. The assembly was placed in a circulating air oven at 180 F. fora period of 4 hours. Thereafter, the laminate assembly was allowed tocool and removed from the oven, the clamps removed and the transparentpanel tested. The finished panel was transparent and displayed excellentoptical qualities with no distortion and the panel contained noobservable imperfections. The glass-stretched acrylic laminate wassoaked for 4 hours at both 65 F. and 180 F. with no observabledetrimental eflect, such as warpage, haze, delamination, or opticaldistortion.

EXAMPLE ll The general procedure outlined in example 1 was used tofabricate another 12 inch X 12 inch flat panel. In this case, the glassface was one-eighth inch electrically conductive coated Pyrex glass andthe interlayer casting resin was a mixture of 350 grams of RTV-602silicone resin and 1.75 SRC05 catalyst.

After mixing, the catalyzed resin was thoroughly evacuated to remove airbefore casting into the panel assembly. The entire assembly was curedfor 24 hours at room temperature and then 24 hours at F. The flat panelexhibited the same type of optical excellence as that of example 1 withan overall light transmittance of 83 percent. The electricallyconductive coating at the inner glass surface maintained essentially thesame resistance and demonstrated adequate deicing characteristics as aresult of powering the coating when the panel had been cold soaked for 4hours at 65 F. Temperature cycling from 4 hours at -65 F. to 4 hours atF. indicated no detrimental efiect to the composite structure.

EXAMPLE [I] The procedure of example 1 was repeated using an interlayersilicone casting resin of RTV-602 and 0.25 parts per hundred of catalystSRC-OS, Both double strength window glass and 0.250 inch plate glasswere laminated with 0.675 inch stretched Plexiglas(polymethylmethacrylate) in the 12 inch X 12 inch configuration to yieldtransparent laminates of good optical quality. The panel with doublestrength window glass face exhibited an overall light transmittance of89 percent, while the panel with annealed plate glass face had a lighttransmittance of about 85 percent.

EXAMPLE IV A full scale windshield was prepared using the foregoingprocedures. The outer windshield glass face was 0.125 inch Pyrex with anelectrically conductive coating and the stretched acrylic member was0.750 inch thick. The interlayer casting resin was about 280 grams ofRTV-602 mixed with 7 grams of SRC-05 catalyst and filled a castingvolume having a thickness of about 0.187 inch. Curing was accomplishedmaintaining the assembly for 16 hours at room temperature and for 6hours at 160 F. in a circulating air oven. The large windshielddisplayed the same desirable optical properties indicated in theexamples H1]. The flat transparent laminate was exposed to soakingtemperature of 65 F. and 175 F. for several cycles without detrimentaleffects of warpage or delamination. The electrically conductive coatingwas powered after the windshield had been soaked at -65 F. temperaturefor 2 hours and exhibited a satisfactory deicing operation.

The panel assembly was again returned to several cycles of 65 F. and 175F. temperature soaking with no impairment of the electrical system orthe optical quality.

EXAMPLE V Another test panel of the same windshield configuration wasconstructed according to the procedure and transparent elements ofexample 1V. After curing, the finished laminate displayed the sameexcellent optical quality and optical transmission properties of theprevious windshield panel. A special check fixture was used to obtaindeflection data at conditions of environmental testing to evaluate thequantitative deflection characteristics. Excellent laminate stabilitywas demonstrated for this comparatively large configuration of stretchedSwedlow 350 and electrically coated glass composite at both -65 F. and175 F. soak conditions. While minor deflection was observed as a resultof the differential coefficients of expansion for the two glazingmaterials, some effect was expected because of the lack of edgerestraint on structural lead ing effect. The stretched Swedlow 350member showed very little flatness deviation as measured across thediagonal dimensions. The center deflected a maximum of 0.030 inch at 65F. and returned to zero deflection at room temperature condition. Thedeflection observed at the glass face was almost negligible for both the-65 F. and 175 F. soak conditions. The effectiveness of the castinterlayer is evident in that the relative movement of the acrylicstructural member with thermal cycling is not transferred to the glassface thereby preventing significant deflection, stress, or failure. Atthe same time, no visible effects of delamination, voids, color oropacity was observed and the transparent laminate retained very goodoptical quality.

It will be readily apparent to those skilled in the art that the presentinvention makes possible the production of glassstretched acryliclaminates having thermal cycle properties unknown to the prior art.Laminates produced according to the present invention having a siliconeresin interlayer have been subjected to thermal soak and shock cycles ofF. to F. without noticeable deflection and with no delamination. Inmarked contrast, glass-stretched acrylic laminates comprisingpolyvinyl-butyral or other previously suggested interlayer materialshave exhibited glass fracture or delamination when subjected to evenless severe temperature conditions.

Thus, the present invention is of particular advantage with regard tothe production of aircraft Windshields and transparencies generallysince glass outer surfaces provide maximum resistance to abrasion andare excellent substrates for electrically conductive coatings which mayfunction as deicing or defogging means and since resin sheets such asoriented acrylic sheets possess fracture propagation resistance(toughness) not obtainable with glass. In addition, oriented acrylicspossess excellent optical quality and result in weight savin s when usedin place of glass. In this regard, it has been foun that testWindshields comprising laminates of the present invention have performedquite successfully in birdimpact testing. In such testing, 4 poundchickens were fired from specially constructed guns using compressed airas the propellant at the Windshields. It was found that the chickenscould be fired at the Windshields at speeds in excess of 435 m.p.h.without failure of the stretched acrylic sheet and with only cracking ofthe outer glass sheet. Bird-impact resistance may, of course, beincreased by increasing the thickness of the stretched acrylic member.

Having fully described the present invention, it is to be understoodthat it is not to be limited to the details set forth, but is of thefull scope of the appended claims.

We claim:

1. A laminate comprising a first skin comprising a glass sheet and asecond skin comprising a tough, hard substantially rigid stretchedacrylic resin and a silicone resin interlayer provided between saidskins and bonded to each of said skins, said interlayer being nonrigid,deformable and flexible.

2. The laminate of claim 1 wherein each of said skins and saidinterlayer are transparent.

3. The laminate of claim 1 wherein said silicone resin interlayermaterial is curable and pourable at :room temperature.

4. The laminate of claim 1 wherein said silicone resin interlayermaterial is catalytically curable.

5. The laminate of claim 1 wherein said silicone resin interlayercomprises methylvinylpolysiloxane and methyl hydrogen polysiloxane.

2. The laminate of claim 1 wherein each of said skins and saidinterlayer are transparent.
 3. The laminate of claim 1 wherein saidsilicone resin interlayer material is curable and pourable at roomtemperature.
 4. The laminate of claim 1 wherein said silicone resininterlayer material is catalytically curable.
 5. The laminate of claim 1wherein said silicone resin interlayer comprises methylvinylpolysiloxaneand methyl hydrogen polysiloxane.